In humans, numerous retinal diseases can damage the light-sensing photoreceptor cells causing permanent blindness. The human retina cannot replace these damaged cells but in the zebrafish retina, photoreceptor injury results in a robust response from intrinsic stem cells (i.e. Mller glia) that regenerate the damaged photoreceptors and restore vision. Other than this striking difference in regenerative ability, human and zebrafish retinas are otherwise similar and contain all of the same basic cell types, including Mller glia. The zebrafish retina is, therefore, an excellent model organism in which to identify the mechanisms that control photoreceptor regeneration, and this information will be crucial for developing regenerative therapies to replace photoreceptors and treat blindness in humans. Recent studies have made exciting progress, utilizing information gleaned from zebrafish studies to stimulate a limited regeneration response in the mouse retina. These are important results showing some regenerative potential in the mammalian retina but, so far, these techniques cannot produce the large numbers of photoreceptors that would be required to treat blindness. To accomplish this will require a much more detailed understanding of the complex molecular pathways involved in photoreceptor regeneration than what currently exists. The overall objective of our research program, therefore, is to fill these knowledge gaps by using zebrafish as a model to understand the mechanisms that govern photoreceptor regeneration. Most of the current understanding of these mechanisms involves gene regulation at the transcriptional level, but recent studies show that microRNAs (miRNAs) can be critical post-transcriptional regulators of neurogenesis. Information around the roles of miRNAs in photoreceptor regeneration is critically lacking but preliminary data show that the microRNA miR-18a plays a vital role. Following photoreceptor injury in [CRISPR/Cas9] miR-18a mutant fish, the retina has twice the number of dividing photoreceptor progenitors and inflammation persists for longer than in wild type fish. Based on these data, the currect objective of the present work is to identify the mechanisms through which miR-18a functions and the central hypothesis to be tested is that following retinal injury, among Mller glia and photoreceptor progenitors, miR-18a regulates key molecular pathways that govern the cell cycle and photoreceptor regeneration. This will be tested by first determining the role of miR-18a in regulating the cell cycle and photoreceptor regeneration (Specific Aim 1), then by identifying specific molecules/pathways that miR-18a regulates (Specific Aim 2) and, finally, by determining if miR-18a regulates photoreceptor regeneration by suppressing inflammatory signaling (Specific Aim 3). This research is innovative because it will be one of the first studies to identify mechanisms through which a miRNA post- transcriptionally regulates photoreceptor regeneration. This work is significant because this understanding is vital to developing therapeutic approaches that efficiently regenerate photoreceptors and restore vision in the human retina.